Fuel cells electrochemically convert fuels and oxidants to electricity, and fuel cells can be categorized according to the type of electrolyte (e.g., solid oxide, molten carbonate, alkaline, phosphoric acid, or solid polymer) used to accommodate ion transfer during operation. Moreover, fuel cell assemblies can be employed in many (e.g., automotive to aerospace to industrial) environments, for multiple applications.
A Proton Exchange Membrane (hereinafter "PEM") fuel cell converts the chemical energy of fuels such as hydrogen and oxidants such as air/oxygen directly into electrical energy. The PEM is a solid polymer electrolyte that permits the passage of protons (i.e., H.sup.+ ions) from the "anode" side of a fuel cell to the "cathode" side of the fuel cell while preventing passage therethrough of the reactant fluids (e.g., hydrogen and air/oxygen gases). Some artisans consider the acronym "PEM" to represent "Polymer Electrolyte Membrane." The direction, from anode to cathode, of flow of protons serves as the basis for labeling an "anode" side and a "cathode" side of every layer in the fuel cell, and in the fuel cell assembly or stack.
Usually, an individual PEM-type fuel cell has multiple, generally transversely extending layers assembled in a longitudinal direction. In the typical fuel cell assembly or stack, all layers which extend to the periphery of the fuel cells have holes therethrough for alignment and formation of fluid manifolds that generally service fluids for the stack. As is known in the art, some of the fluid manifolds distribute fuel (e.g., hydrogen) and oxidant (e.g., air/oxygen) to, and remove unused fuel and oxidant as well as product water from, fluid flow plates which serve as flow field plates of each fuel cell. Also, other fluid manifolds circulate coolant (e.g., water) for cooling. A particular fluid flow plate might be a bipolar, monopolar, combined monopolar (e.g., anode cooler or cathode cooler), or cooling plate.
As is known in the art, the PEM can work more effectively if it is wet. Conversely, once any area of the PEM dries out, the fuel cell does not generate any product water in that area because the electrochemical reaction there stops. Undesirably, this drying out can progressively march across the PEM until the fuel cell fails completely. So, the fuel and oxidant fed to each fuel cell are usually humidified. Furthermore, a cooling mechanism is commonly employed for removal of heat generated during operation of the fuel cells.
It has been recognized that flow field plates are susceptible to dissolution or embrittlement. It has been further recognized that light weight metals such as aluminum and titanium and their alloys provide improved electrical and thermal conductivity over use of graphite in forming flow field plates. Moreover, it has been recognized that the use of these light weight metals in flow field plates can present shortcomings which include a relatively rapid rate of deterioration and/or the formation of oxide films with relatively high electronic resistance.
One known configuration for a fuel cell plate forms a core from aluminum or titanium. A protective coating of stainless steel is placed atop the core. The protective coating is covered with a titanium nitride topcoat having a plurality of defects therein. The defects in the topcoat expose the protective coating to a corrosive operating environment in the fuel cell. The protective coating includes chromium, nickel, and molybdenum for oxidative passivation at the exposed locations in order to protect the core against corrosion. Such a design is disclosed in U.S. Pat. No. 5,624,769 to Li et al. (entitled "Corrosion Resistant PEM Fuel Cell," issued Apr. 29, 1997, and assigned to General Motors Corporation).
However, a need exists for reducing problems associated with corrosion of flow field plates having an exterior which is susceptible to corrosion. A further need exists for reducing damage to the fuel cell assembly because of electrochemical gradients therein.